HOW CAN I FIX THE WATER LEAK ON THORN OLYMPIC WALL MOUNTED GAS BOILER?

Our daughter gets married Saturday week and she and her partner have a problem with their Thorn 'Olympic' Wall Mounted balanced flue Gas Boiler. It's the original boiler in their three bed terraced property -circa late 70's. Drips of water leak from the underside of the bolier. They have been told the boiler is now 'extinct'
so no parts would be available. However, a new unit
could be fitted for £2500. Sorry to be so vague about the boiler id but maybe someone could give guidance for a possible repair?! Thanks- John